1. Field of the Invention
This invention relates to systems that measure the performance of a high-speed data communications channel. More precisely, the preferred embodiment of the present invention is a method and apparatus for identifying a plurality of errors of a multi-valued data signal that occur outside the boundaries of an eye mask as defined by regions above, below and in the center of an eye diagram.
2. Description of the Related Art
In recent years, the performance of a high-speed communication facility or device has risen to a point that the ability of accurately measuring its quality has become an ever-increasing challenge. In the area of data communication devices, for example, efforts to reduce size and power while increasing the throughput of a device has increased the possibility of error. Network suppliers, integrators and users want assurances that these devices will perform reliably and can accommodate data transmission rates that routinely exceed several gigabits/second. Providing such assurance requires determining the effective error rate of either the data received by or data transmitted through a high speed communications channel or device.
One approach of evaluating the performance of a high speed communications channel device or system is to use a bit error rate tester. Here, a predetermined bit pattern is transmitted through the target channel, device or system and on to the bit error rate tester. The difference between the known transmitted bit pattern and the bit stream generated by the target device or system are considered errors and are accumulated by the bit error rate tester to determine the effective bit error rate (i.e., the fraction of the received or transmitted bits that are in error) of the target device or system. Even though bit error rate testing (BER) is a relatively significant measure of performance, merely measuring the error rate of a high speed communications device or system does not provide enough data to characterize the behavior of a communications channel, device or system. As a result, instruments such as a bit error rate testers and oscilloscopes have the functionality of creating an eye diagram to facilitate characterizing a high-speed communications device or system during the various stages of development. Here, a number of voltage levels of the data signal transmitted through a high-speed communications device or system is sampled at specified instances in time. A voltage-versus-time plot of errors known as “events” that depicts the behavior and performance of the high-speed communications component or subsystem is then generated and displayed as an eye diagram, for example.
For production or manufacturing environments, generating a comprehensive series of tests that incorporates generating an eye diagram for each new component or communications subsystem is too costly and quite time consuming. One technique of a more efficient way of validating the performance of a communications device or subsystem provides a quick indication of how a communications device or system performs against an industry develop characterization and performance template commonly referred to as an eye mask. These templates have been created by a number of industry groups to assure the interoperability of components or subsystems that might be configured into a number of different communication systems. To accomplish this, a predetermined, known data signal is transmitted through the target device or subsystem to see the number of times the data signal that is generated by that device or subsystem deviates across the boundary of one or more of the zones as defined by the eye mask. If the data signal from the target device or subsystem does not deviate very many times into one or more of the zones, thereby producing only a limited number of mask violations, it is likely that the target device or subsystem will have little or no trouble operating properly and meeting or succeeding the specification of a particular application.
There are several commercially available bit error rate testing system that characterize and validate the performance of a data signal from a device or communications subsystem using a form of an eye mask or template. Systems such as Agilent Technologies' 81250 as well as the 10 Gb/sec and the older 3 Gb/sec bit error rate testing systems measure the bit error rate of the data signal from a device or communications subsystem at various decision threshold levels and clock-to-data time delays in the eye diagram. The samples of the data signal transmitted through a target device or communications subsystem that are on the wrong side of a mask boundary for the center of the eye are displayed at different bit error rate levels than those that are on the correct side of the mask perimeter.
For contrast, another known method for studying the quality of the signals involved in a digital communications system is to look at a bit error rate eye contour display. FIG. 1 is a diagram of a typical bit error rate eye contour map produced by a prior art bit error rate testing systems. FIG. 1 shows the eye contour that comprises several different bit error rate levels where each level is represented by a different graphic icon. For example, the outside contour 102 shown as a series of black circular icons designates a bit error rate of 1×10E−4 or one error in 10,000 bits. Thus, if the detector is set at this decision level and time delay, a device or communications subsystem transmitting a 2.4 Gigabits per second data signal where 2,400,000,000 bits occur in every second, the expect error rate would be 240,000 errors a second.
This type of view of signal quality is accomplished by making a plurality of bit error rate measurements at various instances in time and voltage threshold. These measurements are then used, along with the theory for randomly distributed errors, to estimate the bit error rate at a given decision threshold by making bit error rate measurements at a series of decision thresholds and extrapolating to get an estimate the bit error rate of those areas that are not actually measured. The problem with this technique is that it may take minutes, if not hours, to complete depending to what bit error rate contour measurement is desired. More importantly, these systems do not do an eye mask test but instead produce only a contour of the bit error rates of a data signal that has been transmitted through a target device or communications subsystem.
More recently, Agilent Technologies has developed a technique that performs mask-like measurements along the inside perimeter of the opening of the eye diagram. Under program control, the 81250 parallel bit error rate testing system measures the bit error rate at a specific number of points inside the opening of the eye in accordance with a supplied set of time-threshold voltage settings or parameters. This defines a set of extreme points or corners of the desired mask template and generates the results in a tabular spreadsheet-like format. Using this technique, the 81250 parallel bit error rate testing system calculates a partial mask violation report only about a polygon shaped area in the center of the eye diagram. With this technique, once the synchronization process is complete, the 81250 test system then employs up to 32 different locations inside the eye opening to form a polygon shaped template and then makes a series of quick bit error rate measurements at those locations. Even though the 81250 test system measures the bit error rate of the date signal for a finite number of points in the center of an eye diagram, it is not able to move the measuring point in areas above or below the eye diagram. As a result the 81250 test system can not measure the violations that might occur when a data signal from a target device or subsystem deviates across the boundaries and into the areas are either above or below the eye as specified by an industry standard eye mask. This is required to detect overshoot and undershoot or other signal quality degradations that manifest themselves above and/or below the eye center which does not necessarily effect the bit error rate of the transmitted signal but may effect other design limits of the device or system.